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Bolt Size and Grade

02-08-2010, 06:44 PM

I machined a plate to mount my 12 Volt electric winch on the tongue of my little trailer.

The mounting studs from the original plate are 3/8 propietary shoulder bolts. I drilled the 3/8 flat bar that I had laying around and machined enough room on the bottom, to accept the lock washer and about half of the nut. That allowed me to use the same bolts, even though the flat bar is thicker than the original plate.

I'm going to weld some 3 in x 1/8 flat bar to each side of the bottom of the plate that I made.

When I mount it to my 3 in. trailer tongue will 3/8 Grade 8 be sufficient to bolt through the plate and the tongue, from the sides?

I will use this 6000 lb (on a rolling load) winch to pull my 500 lb stacker onto the trailer.

Comment

You don't need fine thread or grade 8, 5 are plenty for most DIY work and preferable in a lot of cases. Toss the lock washer to get a full nut. Locks are not needed for most structural work, they been going by the wayside in a lot of applications.

Comment

You'll need a minimum of 3 3/8" bolts due to bolt bearing area failure.

The 3"x1/8" bar you are bolting through is A36 steel, which has an ultimate strength of 58,000 psi. The formula for bolt bearing capacity is:

Bolt diameter*material thickness*ultimate strength*.75

3/8"x1/8"x58,000x.75=2,039lb capacity per bolt.

So to equal the 6,000lb capacity of your winch (because you want the winch to break before your bolts tear out) you need a minimum of 3, and they don't have to be grade 8. It doesn't matter how strong the bolts are if the base metal tears way before the bolts approach their full strength.

It appears that chart shows the breaking strength, not the safe load. Also, the values for shear are for single shear plane failure.

Keep in mind that the maximum strength will result when shear or tension acts through the bolt. Any eccentricity causes a prying action that increases the stress on the bolt.

80% of failures are from 20% of causesNever compromise your principles today in the name of furthering them in the future."All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko"We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal"Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal

Comment

I would go with the Grade 8 just for safety sake. Grade 2 bolts shouldn't be considered for load applications. Grade 5 would work but they have a tendency to stretch over time. The Grade 8 will not stretch. Washers and lock washers will also help in getting the right tension on the bolted connection without damage to the frame and plates being bolted together and protect the bolt and nut from damage.

While studying Mechanical Engineering, in my Materials Design Class, we spent a great deal of time working the calculations and theory behind bolted connections and using the right bolt by size and design for the job. Someone already did all the math, that's why we have charts and tables for selecting the right bolt size based on tension and shear loads in a bolted connection. The washer and lock washer play into the preload tension and the final torque of the bolted connection as well as protecting the metal around the hole of the bolted area.

Believe it or not, a Grade 5, 1/4" bolt can hold a F-150 Pickup (Base Model, Short bed, limited options) in the air by tension load alone. However, the Safety Factor is almost 0 and any shear load (side pressure, perpendicular to the tension load) would snap the bolt.

SAE Threads (Fine) is overkill unless you are worried about vibration causing the nut to come loose under load. Standard (Course) Thread is usually sufficient. 3/8"-16 Grade 8 with washers and lock washer would give the best bolted connection for you purpose. Safety Factor will be maximized and bolt stretch minimized. You could use LockTite on the bolts, but then you would have trouble getting the nuts and bolts undone in the future without having to apply heat first to the nuts to break them loose.

Depending on how your mount is designed, you will most likely see more shear load on the bolts than tension load. The maximum tension load will be on the bolts on the backside of the winch due to cable wrap over the drum.

Comment

Yielding (stretching irreversibly) will happen if they are stressed beyond the yield point. Yes, grade 5 bolts have a slightly lower yield point. But that is irrelevant.

Bolting through a 1/8" sheet it's not going to matter if they are grade 8, 5 or even 2 because bolt bearing area is the weakest link.

If you took two sheets of 16ga metal and bolted them together with a 1" bolt, it wouldn't matter if it were grade 8, 5, or even 2. Heck, it could just about be a plastic bolt... the base metal will fail at the bolt bearing area.

A connection is only as strong as its weakest link and in this case the weakest link is the base metal. You can make it the connection stronger by using larger diameter bolts, larger number of bolts, or a thicker plate (all of which increase bolt bearing area), but using higher grade bolts will not make the connection any stronger. This is sort of like replacing a class II hitch with a class III hitch "for safety's sake" so you can tow a car with a shoelace. It did nothing to increase the allowable load for the system.

80% of failures are from 20% of causesNever compromise your principles today in the name of furthering them in the future."All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko"We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal"Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal